Ziegler-Natta type catalysts are well known and have been used since the early 1950's. Generally, the catalyst comprises a transition metal compound, typically titanium in the 3 or 4 valence state (e.g. TiCl3 or TiCl4) supported on magnesium chloride.
In some cases, the catalysts are prepared in conjunction with an electron donor (ED), which may be used to dissolve the MgCl2 and TiCl3 when they are used. This type of formulation teaches away from the current disclosure. When supported on silica, and placed in a gas phase reactor with at least one co-catalyst, typically an aluminum compound such as a trialkyl aluminum (e.g. triethylaluminum (TEAL)) or an alkyl aluminum halide (e.g. diethylaluminum chloride (DEAC)) this combination makes a good catalyst for the polymerization of ethylene. When the ED is used as the solvent in the formulation it tends to narrow the molecular weight distribution (MWD—weight average molecular weight/number average molecular weight (Mw/Mn)) of the resulting polymer. As the ED is difficult to remove, polymers having a broader MWD are not easy to manufacture using this catalyst synthesis procedure. Also, as the ED must be capable of dissolving the MgCl2, and in some cases TiCl3, the choice of the electron donor is limited, a problem which is solved with the current disclosure. A good description of these types of catalysts is contained in U.S. Pat. No. 4,302,566 issued Nov. 24, 1981 to Karol et al., assigned to Union Carbide Corporation.
Alternately, the catalysts may contain magnesium chloride, which may be generated from a dialkyl magnesium compound, and compounds containing an active chloride. These types of catalysts are described in U.S. Pat. No. 4,324,691 issued Apr. 13, 1982 to Hartshorn et al. assigned to Imperial Chemical Industries. This is a broad patent covering the synthesis of these types of catalysts but not disclosing the process of the present invention.
In a similar vein, U.S. Pat. No. 5,633,419 issued May 27, 1997 to Spencer et al. assigned to The Dow Chemical Company discloses a supported Ziegler-Natta type catalyst or catalyst precursor. However, it is a limitation of the reference that the halide used is hydrogen halide (e.g. HCl). The reference teaches away from the subject matter of the present invention, which preferably uses alkyl halides as the halide source. Additionally, the reference teaches the use of an agglomerated support such as silica, instead of a typical granular support, which is contrary to the method for preparing the catalyst of the present invention.
European Patent Application 0 744 416 published Nov. 27, 1996 in the name of BP Chemicals Limited (BP) teaches supported catalysts and catalyst precursors. The reference teaches that the support is reacted with either an aluminum compound or a magnesium compound followed by an alkyl halide and a tetravalent titanium compound. Electron donors can be added to the support at anytime during the synthesis. The reference fails to teach that ketones could be used as electron donors. This reference also teaches that the Al:Mg molar ratio is between 0.05 to 0.35, which is outside the scope of the present catalyst formulations. This reference also teaches that the halogenating agent must be used in a molar ratio such that all of the Mg and Al compounds are converted to their respective halides, a limitation which is divergent from the present invention. That is, in accordance with the present invention, there is an excess of reactive aluminum and/or magnesium groups over reactive halide so that not all of the alkyl aluminum or magnesium are reacted to their respective halides (chlorides).
BP patent EP 0 595 574 B1 granted in Jan. 2, 1997 requires that the catalyst be prepared on a support treated with an organosilicon compound. The supports used in accordance with the present invention are treated thermally and chemically (aluminum alkyls) but not with organosilicon.
BP patent EP 0 453 088 B1 granted in Jul. 31, 1996 teaches that the catalyst be made on dehydrated silica and that the catalyst needs to be pre-polymerized to obtain a useful catalyst for the synthesis of linear low density polyethylene (LLDPE). The catalysts of the present invention do not need to be pre-polymerized to be useful for the polymerization of ethylene to manufacture polyethylene. This reference also teaches against the addition of a second aluminum compound. While the patent teaches about low levels of electron donors (e.g. ED:Ti<0.2) it fails to teach that ketones are useful as electron donors.
NOVA Chemicals Ltd. U.S. Pat. No. 6,140,264 issued Oct. 31, 2000 and U.S. Pat. No. 6,046,126 issued Apr. 4, 2000 to Kelly et al. both deal with making a TiCl4 supported catalyst on magnesium chloride (precipitated from a dialkyl magnesium compound and an organic halide) which is on a thermally and chemically treated silica. This reference teaches against the addition of a second aluminum compound and limits the electron donor to relatively low levels both restrictions, which have been removed from the present invention.
The use of ketones in Ziegler-Natta catalysts has been documented in the patent literature. For example U.S. Pat. No. 4,324,691 to Hartshorn assigned to Imperial Chemical Industries, issued on Apr. 13, 1982, U.S. Pat. No. 6,187,866 to Jorgensen assigned to Union Carbide Chemicals and Plastics, issued Feb. 13, 2001, U.S. Pat. No. 6,174,971 assigned to Fina, issued Jan. 16, 2001 and 2005/0288460 assigned to Engelhard Corporation published Dec. 29, 2005 all discuss the use of ketones, among other compounds as electron donors useful in the synthesis of Ziegler-Natta catalysts. However, none of these patents identify the subject matter of this disclosure.
In U.S. Pat. No. 4,496,660 issued to Dow Chemical Company on Jan. 29, 1985, the electron donor (ED) is added directly to the alkyl magnesium compound, to pacify it prior to adding the transition metal compound, which is not a requirement of the current invention. In this patent a second transition metal containing an O or N compound when used in conjunction with another transition metal compound was found to alter the molecular weight of the polymer, a restriction that is not required in the current patent (column 11, L 55). Data on seven different electron donors including acetone are presented in Table 1 to illustrate their invention. However, due to the number of variables changed in the experimentation process (i.e. ED:Ti, Mg:Ti and Mg:Al) this patent does not teach or suggest the present invention.
In another application, that is the high pressure, high temperature, free radical process used to manufacture low density polyethylene as described in US 2004/0210016 issued to BASF on Oct. 21, 2004, ketones have been identified as being useful molecular weight regulators a function which is not performed in the present invention.
The use of electron donors has also been discussed in the open literature as well. For instance Mori et al. in Journal of Molecular Catalysis A: Chemical, 140, 1997, 165-172, showed that adding either ethyl benzoate or dibutylphthalate to a catalyst by supporting titanium on MgCl2 resulted in increasing the Mn of polypropylene made with these catalysts. This teaching is against the subject matter of the present specification, which teaches reducing the Mn of the polymer when using a ketone. Gao in Polymer 45, 2004, 2175-2180 indicated that hydrogen response in polypropylene varied with the type of internal donor but again did not discuss the subject matter disclosed herein. Spitz, in Studies in Surface Science and Catalysis 1990, 59, 117-130 showed that for ethylene polymerization, the type of electron donor had a large effect on the melt index. The data presented in this paper showed that when using triethyl aluminum as the co-catalyst, the highest melt index polymers were obtained using diphenylether while the lowest melt index polymers were obtained using diphenyl dimethoxy silane. The findings presented in this paper do not disclose or suggest the subject matter of the present specification. Recent work by Chadwick et al. in Macromol. Chem. Phys. 197, 1996, 2501-2510 showed that in the synthesis of isotactic polypropylene catalysts made with diether type internal electron donors showed improved hydrogen response when compared to catalysts made with ester containing systems. Again, this teaches away from the present invention.
The present invention seeks to provide a process for the polymerization of olefin monomers preferably ethylene in which the hydrogen response of the catalyst is significantly changed compared to a generally known Ziegler Natta catalysts. Prior to the discovery of the present patent application, increasing the H2 in the reactor generally reduced the molecular weight of a polymer. To a limited extent increasing the polymerization temperature or increasing the level of co-catalyst in the reactor could also affect the molecular weight of the resulting polymer. All of these methods have some drawbacks.
For instance, increasing the H2 in the reactor tends to reduce catalyst productivity via two processes. First, for catalysts useful for ethylene polymerization, hydrogen is known to deactivate the catalyst as was reported by Garoff et al. in European Polymer Journal 38, 2002, 121 to 132 and Huang, et al. in Ind. Eng. Chem. Res. 1997, 36, 1136 and second increasing the level of H2 in the reactor reduces the partial pressure of ethylene which also reduces the catalysts productivity. Hence, when a high MI grade (5 to 125) such as is used in injecting molding grades is required to be manufactured the catalyst productivity can be substantially reduced resulting in higher costs to the producer.
Problems associated with increasing the temperature revolve around reducing the operating window in which a commercial reactor can safely operate as discussed by Seville in Powder Technology, 97(2), 1998, 160-169. At the commercial scale, a sudden loss of fluidization can cause the granular resin in the reactor to agglomerate resulting in unwanted downtime required to clean the reactor.
Increasing the co-catalyst is also known to reduce the molecular weight of the resin however it has also been shown to reduce the resin's physical properties as reported by NOVA Chemicals in U.S. Pat. No. 6,825,293 issued on Nov. 30, 2004.
The inventors have unexpectedly found that simply by using a ketone as the electron donor in a Ziegler-Natta catalyst, polyethylene having lower molecular weight can be easily produced.